1. Technical Field
The present disclosure relates in general to methods of drilling wellbores, for example, but not limited to, wellbores for producing hydrocarbons from subterranean formations, and more particularly to methods of measuring annulus drilling mud flow rate, either during drilling of a wellbore or during periods of fluid flow only.
2. Background Art
Much work has been done in the industry over the past several decades to measure the rate of flow of fluids within the annulus of a well. Some workers in the field (see for example U.S. Pat. Nos. 6,938,458; 6,672,163; 6,817,229; 6,829,947; and 6,378,357, (Han, et al., assigned to Halliburton)) have claimed that a change in annular mud velocity will occur at the point where lost circulation is occurring. However, these references do not disclose computing volumetric flowrate of mud in the annulus, rather the patents focus on measuring axial, radial, and tangential velocities of the mud to determine if a kick has occurred (potentially dangerous situation where formation fluids flow into the well displacing the mud and altering the hydrostatic pressure which the mud creates to combat flow into the well). Other work has focused on identifying a kick by monitoring flow rates. See for example U.S. Pat. No. 4,527,425, Stockton, who discusses using Doppler Effect to measure changes in ratio of incoming to output mud flow rate. The occurrence of a change in this ratio above a preselected value will trigger an alarm indicative of the commencement of either a rapid influx of fluids from the formation into the mud stream (blow-out) or a rapid outflow of mud into the formation (lost circulation). Stockton does not, however, discuss measuring volumetric flow rate of the mud, only “flow rate”, which Stockton defines as the rate of drilling fluid flow toward (or away from) the drill bit as a function of time. In fact, Stockton teaches to avoid the necessity of making substantial and complicated calculations to compensate for riser pipe volume variations (for example as would be necessary when the measurement is made near the surface in telescoping riser pipes) where the mud flow enters and leaves the riser pipe.
Various apparatus and methods are described in these references for obtaining the information. For example, the '163 patent mentions that by operating transducers at multiple frequencies, fewer transducers are needed to generate frequency dependence data. For example, a system might include a “1 MHz transducer” operated at 1 MHz and 3 MHz and a “9 MHz transducer” operated at 9 MHz and 27 MHz. The '163 patent also explains that speed of sound in the fluid can be calculated by measuring the time of flight of the pulse over the known distance between a transmitter and receiver. The receiver may also be used to determine the attenuation coefficient of the fluid, preferably at multiple frequencies (including third harmonics), by measuring the decay of multiple reflected signals, or comparing the transmitted signals to those of a fluid with known attenuation coefficient. As explained in the '357 patent, apparatus may use ultrasonic signals to measure Theological properties of a fluid flow such as, e.g., the consistency index K, the flow behavior index n′, the yield stress, or other parameters of any given model for shear rate dependent viscosity. In one described method embodiment, the method includes: (a) transmitting an acoustic signal into the fluid flow; (b) receiving acoustic reflections from acoustic reflectors entrained in the fluid flow; (c) determining a Doppler shift of the acoustic reflections in a set of time windows corresponding to a set of desired sampling regions in the fluid flow; and (d) analyzing the Doppler shifts associated with the set of sampling regions to determine one or more Theological properties of the fluid flow. As is known, the frequency shift caused by motion of the fluid is proportional to the velocity of the fluid, and this allows the construction of a velocity profile of the fluid flow stream.
Despite these efforts, it has become evident that the measurement of mud velocity will work to recognize lost circulation or influx events only if the hole is constant diameter, the pipe is in a consistent position within the well, and the detectors are consistently aimed into common hole sectors. However, the realistic situation in wells causes these limitations to lead to misleading conclusions on the root cause, location, and appropriate treatment, and cost well operators considerable expense. Drill pipe is not normally centralized above the bottom hole assembly. Excentralization of the drillpipe will lead to variations in the annular velocity around the pipe. Detectors aimed in different radial directions will detect different velocities. As recognized by Priest, “Computing Borehole (BH) Geometry and Related Parameters From Acoustic Caliper Data,” SPWLA 1997 G, borehole eccentricity, major and minor diameters, elliptical orientation, eccentering radius, and “direction” (position of tool relative to center of borehole) are important considerations. Priest describes use of an elliptical model of the BH to correct acoustic travel times and “radius images”, which can be generated from the travel times. Using the “eccentering data” it is possible to construct a “centered” radius image (i.e., image that would have been obtained had the acoustic transmitter been centered). A rotating acoustic sensor is disclosed. There is, however, no disclosure of using caliper to calculate local mud flow rate at points in the wellbore.
Zemanek et al., “The Operational Characteristics of a 250 KHz Focused Borehole Imaging Device”, SPWLA 1990, presents experimental data on properties of oil-based muds (OBM) obtained at different temperatures and pressures, and forms a nomograph defining the radial operating range of a particular acoustic instrument in these muds. The method and apparatus described (termed a “circumferential borehole imaging log” by the authors) employs two concave acoustic transducers having different focal points that rotate through 360° to obtain time of flight measurements. A separate mud flow velocity transducer is mounted in a cavity open to the borehole. Together they allow caliper to be determined by the equation caliper=velocity×time. However, there does not appear to be discussion of calculation of local, temperature and pressure corrected mud flow rate from an acoustic caliper and velocity of the mud measured acoustically. Rather, the authors focus on determining the limitations of the device in OBMs. Importantly, in discussing FIGS. 5A and 5B, they conclude that “acoustic attenuation” in an 11 ppg OBM is “a complicated function of temperature and pressure”. For a 15 ppg OBM it is “even more complicated.” (This does not even take into consideration gas, rock cuttings, and other material in the OBM.) In short, their statements and figures indicate that acoustic measurements in these muds may be “unpredictable” due to the unpredictable nature of acoustic attenuation. This “unpredictability” increases as density of the mud increases. (On the other hand, acoustic velocity in OBM seems quite predictable as a function of temperature (T) and pressure (P)—see FIGS. 4A and 4B).
Several SPE papers discuss how acoustic attenuation of muds and mud acoustic velocity depend on many parameters. For example, Maranuk, in SPE 38585 (1997), discloses that acoustic velocity in mud depends on mud type, density, salinity, T, P, amount of gas and solids in the mud, and discloses a semi-empirical method whereby a dataset of these changes is produced, allowing “on-the-fly” corrections.
It would be advantageous if caliper and fluid velocity measurements could be combined to determine the actual flow rate in the annulus past a point in a well. This would allow integration of the fluid velocity as a function of the hole size around the well to account for pipe position. This would provide true flow rate which then could be used to reliably find a point of lost circulation or a well fluid influx which would then result in the correct diagnosis of the root cause, selection of the appropriate treatment and placement of that treatment where the problem has developed. The methods and apparatus of the present disclosure are directed to these needs.